1. Field of the Invention
The present invention relates to a layered rock salt-type oxide particle powder and a process for producing the same, and in particular to a lithium nickel-layered rock salt oxide particle powder, a lithium cobalt-layered rock salt-type oxide particle powder or a particle powder of a solid solution of these layered rock salt-type oxides, having large charging and discharging capacities with a less amount of adsorbed water in the surface of the particle and with less cycle deterioration, which is particularly useful as a positive electrode active material in a lithium battery, as well as a process for producing the same.
2. Description of the Prior Art
As the development of portable devices such as personal computers, portable telephones, etc. advances in recent years, there is an increasing demand for batteries as a power source. In particular, lithium batteries are extensively studied in various fields because lithium is a substance having a low atomic weight and a high ionization energy and can thus be expected to provide batteries with high electromotive force and high energy density.
Lithium nickel-layered rock salt-type oxides, lithium cobalt-layered rock salt-type oxides and solid solutions thereof or lithium manganese spinel oxides, which are capable of generating a high voltage of about 4 V, are extensively studied as positive electrode active materials used in the lithium battery. These compounds such as layered rock salt-type oxides and spinel oxides are obtained by mixing a starting oxide powder containing nickel, cobalt or manganese with a lithium compound powder and then calcinating the mixture. In particular, lithium cobalt-layered rock salt-type oxides, lithium nickel-layered rock salt-type oxides or solid solutions thereof are known to have higher energy density and better cycle characteristics than those of lithium manganese spinel oxides.
These positive electrode active materials in the form of powder are dispersed in a binder, coated on a metal plate such as copper, and dried for use as a positive electrode in a battery, wherein it is important that the particle-shape and particle size are well-regulated because, as the packing of the particle powder in the coating is increased, the capacity of the resulting battery is raised.
A non-aqueous organic electrolyte is used in a lithium ion battery, but it is said that if a trace of water is present in the electrolyte, the electrolyte is electrolyzed to generate a gas such as carbon dioxide (Denki Kagaku No. 11 (1994) 1023-1029), and in this case, there is a possible danger of the battery cell breakage. Further, if lithium nickel-layered rock salt-type oxides are used in such a cell, their characteristics in the battery are easily affected by the production environment such as humidity, etc., and therefore, it is said that it is necessary to control drying conditions strictly (the Japanese Society of Electric Information Communication, Technical Study Report EE98-72 (1999-01) 57-66). Generally, a powdery material has some water adsorbed in the surface thereof, but according to the above literature, it is considered that residual water in the particle powder as the positive electrode material should also be minimized.
Under the background described above, as particle powders of lithium cobalt-layered rock salt-type oxides, lithium nickel-layered rock salt-type oxides and solid solutions thereof acting as material powders for positive electrode active materials, there is a need for layered rock salt-type oxide particle powders having a narrow distribution of particle sizes with the least amount of water remaining in the surface of the particle, as well as a process for producing the same.
An object of the present invention is to provide particle powders of lithium cobalt-layered rock salt-type oxides, lithium nickel-layered rock salt-type oxides and solid solutions of these rock salt-type oxides having a narrow distribution of particle sizes with the least amount of water remaining in the surface of the particle.
Another object of the present invention is to provide a process for producing the above-mentioned particle powders of lithium cobalt-layered rock salt-type oxides, lithium nickel-layered rock salt-type oxides and solid solutions of these rock salt-type oxides.
Further objects and advantages of the present invention will become apparent for those skilled in the art from the detailed description and explanation given below.
As a result of an extensive series of studies in view of the problems described above, the present inventors have found out that a layered rock salt-type oxide particle powder whose surface has been rendered substantially hydrophobic by coating it with a coupling agent having both a hydrophobic group and a hydrophilic group, particularly a layered rock salt-type oxide powder particle obtained in a specific method, can solve the problems described above, thereby arriving at the present invention.
That is, the first aspect of the present invention is directed to a lithium nickel-layered rock salt-type oxide particle powder, a lithium cobalt-layered rock salt-type oxide particle powder, or a particle powder of a solid solution of these layered rock salt-type oxides, wherein the surface of the particle is rendered hydrophobic by coating it with a coupling agent having both a hydrophobic group and a hydrophilic group.
The second aspect of the present invention is directed to a process for producing a lithium nickel-layered rock salt-type oxide particle powder, a lithium cobalt-layered rock salt-type oxide particle powder, or a particle powder of a solid solution of these layered rock salt-type oxides, which comprises the steps of:
mixing each particle powder of nickel oxide, cobalt oxide, or a solid solution of these oxides with a lithium compound,
allowing the mixed powder to incorporate 1 to 10% by weight of water and compression-molding it to form a molded body having a molding density of not less than 2 g/cm3,
calcining the molded body at 600 to 900xc2x0 C. in an oxygen-containing gas to form a lithium nickel-layered rock salt-type oxide,
a lithium cobalt-layered rock salt-type oxide or a solid solution of these layered rock salt-type oxides,
dispersing the layered rock salt-type oxide in an organic solvent, and
adding a coupling agent to this dispersion in order to coat the surface of the particle therewith.
In a preferred embodiment of the present invention, each particle powder of nickel oxide or cobalt oxide is obtained by pyrolyzing at least one member selected from the group consisting of nickel- or cobalt-containing oxalate, acetate and carbonate at 350 to 500xc2x0 C. in an oxygen-containing gas.
In a preferred embodiment of the present invention, each particle powder of the solid solution of nickel oxide and cobalt oxide is obtained by pyrolyzing at least one member selected from the group consisting of nickel- and cobalt-containing oxalate, acetate and carbonate at 350 to 500xc2x0 C. in nitrogen or an inert gas.
In a preferred embodiment of the present invention, the particle powder of cobalt oxide is obtained by hydrothermal synthesis from a suspension containing cobalt hydroxide in the presence of an oxygen-containing gas.
In a preferred embodiment of the present invention, the mixing ratio of each particle powder of nickel oxide, cobalt oxide or a solid solution of these oxides and the lithium compound is in the range of 1.00 to 1.20 in terms of the molar ratio (Li/M) of lithium to nickel, cobalt, or the total of nickel and cobalt.
In a preferred embodiment of the present invention, the amount of the coupling agent for treatment is in the range of 0.1 to 5.0% by weight to the layered rock salt-type oxide particles.
Each powder of nickel oxide, cobalt oxide and a solid solution thereof (also referred to hereinafter as transition metal oxide) used in the present invention is not particularly limited. The nickel oxide and cobalt oxide are preferably those obtained by pyrolyzing nickel- or cobalt-containing acetate, oxalate, carbonate, etc. at 350 to 500xc2x0 C. in an oxygen-containing gas preferably in the air or oxygen, and the solid solution of nickel oxide and cobalt oxide is preferably the one obtained by pyrolyzing nickel- and cobalt-containing acetate, oxalate, carbonate, etc. at 350 to 500xc2x0 C. in an inert gas such as nitrogen or argon. Further, the cobalt oxide can also be obtained by hydrothermal synthesis from a suspension containing cobalt hydroxide in the presence of an oxygen-containing gas.
If the pyrolysis temperature is less than 350xc2x0 C., the pyrolysis of a powder of a salt containing nickel and/or cobalt (also referred to hereinafter as transition metal) is not satisfactory, thus making it difficult to obtain an oxide particle powder having a uniform composition, while if it is more than 500xc2x0 C., the reactivity of the resulting oxide particle powder is lowered.
Then, the transition metal oxide particle powder is mixed with a lithium compound. The mixing ratio of the lithium compound to the transition metal oxide particle powder, in terms of the molar ratio of lithium to transition metal (Li/M), is preferably in the range of 1.00 to 1.20. If the amount of lithium is lower than this range, then non-positive electrode materials such as nickel oxide, cobalt oxide or solid solutions thereof remain besides the desired layered rock salt-type oxides. These non-positive electrode oxides are difficult to be removed, and thus when a positive electrode is composed of such a particle powder containing them, an excellent battery characteristic, that is, an electrochemical activity in an electrolyte having lithium ion electrical conductivity, is hardly obtainable. On the other hand, if lithium is contained in a larger amount than said range, non-positive electrode active materials such as lithium carbonate, etc. occur in addition to said layered rock salt-type oxides. Removal of these impurities is also very difficult, and thus when a positive electrode is composed of this powder, excellent battery characteristics and electrochemical activity are also hardly obtainable.
The lithium compound used in the present invention includes lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydroxide monohydrate, and these are used alone or in combination thereof.
Then, 1 to 10% by weight of water is contained in the mixed powder consisting of the transition metal oxide particle powder and the lithium compound, and this mixed powder is compression-molded by an extrusion molding machine, a roller compactor or a disk pelleter, to form a molded body having a molding density of not less than 2 g/cm3, and then calcined in an oxygen-containing gas, e.g. the air. If the amount of water in the mixed powder is less than 1% by weight, the strength of the molded body is not satisfactory, thus making handling difficult, and the compressed density in the molded body is not uniform, and this causes a broader distribution of particle sizes of the layered rock salt-type oxide particle powder obtained by grinding after calcination. On the other hand, if it exceeds 10% by weight, the water-soluble lithium compound is readily eluted to change the composition, and thus the qualities of the layered rock salt-type oxide particle powder are made unstable.
Further, if a molded body having a molding density of less than 2 g/cm3 is calcined, the growth of grains of the layered rock salt-type oxide is not satisfactory so that upon forming into a coating film, it produces a film with insufficient packing. The upper limit of the molding density is not particularly limited, but because too high density makes production difficult, its upper limit is usually 3.5 g/cm3, preferably 2.5 g/cm3 or so.
The above molded body is calcined in an oxygen-containing gas, preferably in the air. The calcination temperature is in the range of 600 to 900xc2x0 C., and the calcination time is usually in the range of 2 to 10 hours. If the calcination temperature is less than 600xc2x0 C., the molded body should be calcined for a longer time in order to achieve a single phase of the layered rock salt-type oxide. Further, it produces particles with poor crystallinity, thus failing to exhibit satisfactory battery characteristics. On the other hand, if the calcination temperature exceeds 900xc2x0 C., a part of the layered rock salt-type oxide is decomposed or a part of lithium is lost by evaporation, and thus a particle powder showing sufficient battery characteristics cannot be obtained.
Thereafter, the layered rock salt-type oxide is dispersed in an organic solvent, and a coupling agent is added to this dispersion in order to coat the surface of the particle therewith. Because it is desired that the surface of the particle is coated uniformly, the coating thereof with the coupling agent is conducted preferably not in a dry system but in a wet system, that is, in an organic solvent.
The amount of the coupling agent for treatment may be an amount enough to render the surface of the particle hydrophobic, and this amount is usually in the range of 0.1 to 5.0% by weight to the particle. If the amount of the coupling agent is less than 0.1% by weight, the amount of the coupling agent coated is so small that its effect is not satisfactory. On the other hand, if the amount of the coupling agent for treatment exceeds 5.0% by weight, the efficiency is deteriorated by its excess amount, and thus too large amount of the coupling agent for treatment is meaningless,
The coupling agent used in the present invention includes titanate coupling agents such as isopropyltriisostearoyl titanate, isopropyltridecyl benzene sulfonyl titanate, isopropyl tris(dioctylpyrophosphate)titanate, tetra(2,2-diallyloxymethyl-1-butyl)bis(ditridecyl)phosphite titanate and bis(dioctylpyrophosphate) oxyacetate titanate, bis(dioctylpyrophosphate)ethylene titanate as well as silane coupling agents such as xcex3-chloropropyltrimethoxysilane, xcex3-methacryloxypropyltrimethoxysilane and xcex3-glycido xypropyltrimethoxysilane, and these are used alone or in combination thereof.
The organic solvent used in the present invention includes toluene, methyl ethyl ketone, methyl isobutyl ketone, etc., and these are used alone or in combination thereof.
Prior to dispersion of the layered rock salt-type oxide, the molded body of the layered rock salt-type oxide may be ground and dispersed in an organic solvent, or otherwise the molded body may be ground in an organic solvent in a wet system.
The layered rock salt-type oxide particle powder of lithium nickel, lithium cobalt or a solid solution thereof, obtained in the manner described above, has been rendered hydrophobic by uniformly coating the surface of the particle with the coupling agent. Accordingly, the amount of water adsorbed in the surface of the particle is so small that this product has high charging and discharging capacities with less cycle deterioration and without any adverse effect of adsorbed water on the charging and discharging capacities, cycle characteristics, etc. and is particularly useful as a positive electrode active material in a lithium battery.
The most important feature of the present invention is that the layered rock salt-type oxide particle powder has been rendered substantially hydrophobic by coating the surface of the particle with a coupling agent having both a hydrophobic group and a hydrophilic group.
Another important feature of the present invention is the fact that a transition metal oxide particle powder is mixed with a lithium compound, then this mixed powder is allowed to incorporate 1 to 10% by weight of water and compression-molded to prepare a molded body with a molding density of not less than 2 g/cm3, then calcined at 600 to 900xc2x0 C. in an oxygen-containing gas to form a layered rock salt-type oxide, and thereafter said layered rock salt-type oxide is dispersed in an organic solvent followed by adding a coupling agent to this dispersion to coat the surface of the particle uniformly therewith, whereby the desired layered rock salt-type oxide particle powder having a narrow distribution of particle sizes with a minimum amount of residual water in the surface of the particle can be obtained.
It is considered that a solid state reaction during calcination proceeds generally through mutual diffusion of starting powder particles at a point where they are contacted with each other. The present inventors think that in the case of the lithium compound and the transition metal oxides, the reaction proceeds mainly through diffusion of lithium into the oxide particles because the melting point of lithium is considerably lower than the melting points of these oxides, to permit the diffusion of lithium to occur more readily than diffusion of the transition metals such as nickel and cobalt. On the basis of this thought, selection of the oxide particles is more important than selection of the lithium compound, and the present inventors considered that by this selection, the desired layered rock salt-type oxide having a well-regulated distribution of particle sizes can be formed.
Then, a particle powder of nickel oxide, cobalt oxide or a solid solution thereof synthesized particularly in a specific process was used as a starting transition metal as a result of their extensive examination, whereby the reaction thereof with lithium rapidly proceeded upon calcination, that is, the reactivity of the starting transition metal was enhanced, thus succeeding in formation of the layered rock salt-type oxide with a well-regulated distribution of particle sizes.
The reason for the foregoing is not yet clarified, but is possibly because the purity of the particle powder of transition metal oxides prepared in this process is extremely high while the concentration of impurities such as alkali metals, alkaline earth metals, etc. is significantly low.
Further, it is considered that the layered rock salt-type oxide particles with a well-regulated distribution of particle sizes can be formed because the starting powder is highly reactive as described above and because the layered rock salt-type oxide is prepared by adding 1 to 10% by weight of water to the mixed powder and compression-molding this powder to prepare a molded body having a molding density of not less than 2 g/cm3. If water-free dry powder is used, the particle powder will hardly slide in compression molding, thus making it difficult to transmit compressing pressure uniformly through the system, resulting in non-uniform degree of compressed density. By incorporating a specific amount of water in the system, the particle powder will easily slide to permit compressing pressure to be uniformly transmitted through the system, thereby providing the resulting molded body with uniform compressed density so that a product obtained by calcining and grinding this molded body provides the layered rock salt-type oxide particles with a well-regulated distribution of particle sizes.
The layered rock salt-type oxide thus obtained has a less water content just after calcination, but thereafter it gradually absorbs water from the air atmosphere, resulting in the presence of remaining water in a considerable amount. Accordingly, the layered rock salt-type oxide particles thus obtained are dispersed in an organic solvent, and a coupling agent is added to this dispersion in order to coat the particles uniformly therewith, whereby the layered rock salt-type oxide particle powder having a less content of remaining water in the surface of the particle while maintaining the particle size and particle distribution can formed. The reason speculated for this is that hydrophilic groups in the coupling agent undergo substitution reaction with hydroxyl groups on the surface of the layered rock salt-type oxide particles and the oxide particle surface adsorbs the coupling agent thereon with the hydrophobic groups in the coupling agent orientated to the outside of the particles, and these hydrophobic groups render the surface of the particles hydrophobic, thus substantially preventing adsorption of water from the air atmosphere and minimizing residual water on the surface.